1. Field of the Invention
The present invention relates to a clock changeover apparatus for changing over a plurality of clock signals.
2. Description of the Prior Art
Conventionally, when mediums having different recording and reproduction densities are recorded and reproduced by a single apparatus, clocks corresponding to kinds of the mediums, respectively are required to be provided. Meanwhile, in the case of a disk, a zone division procedure is also employed in which the disk is divided into a plurality of annular zones so as to improve its recording and reproduction densities and recording and reproduction are performed by changing the recording and reproduction densities at each of the zones. At this time, a plurality of clocks corresponding in number to the zones are required to be provided. Thus, a clock changeover apparatus for changing over the clock signals is put to practical use and known.
One example of the known clock changeover apparatus is described with reference to FIG. 14, hereinbelow. In FIG. 14, first, second and third clocks 21, 22 and 23 are different from one another. Each of the first, second and third clocks 21, 22 and 23 is formed by a crystal oscillator. The known clock changeover apparatus includes a changeover switch 24 and a central processing unit (CPU) 25. The changeover switch 24 changes over the first, second and third clocks 21, 22 and 23 so as to generate an output. The CPU 25 supplies to the changeover switch 24 a clock changeover signal 200 indicating to which one of the clocks 21-23 the changeover switch 24 should be changed over. To this end, a necessary number of lines carrying the clock changeover signal 200 are connected between the clock changeover switch 24 and the CPU 25. In case the three clocks 21-23 are provided as shown in FIG. 14, at least two lines are required for discerning three states. Alternatively, one line is used for sending data serially, while the other line is used for sending a clock signal for sampling the serially sent data. Also in this case, a plurality of states can be discerned by a total of two lines carrying the clock changeover signal 200.
Operation of the known clock changeover apparatus of the above described arrangement is described with reference to FIG. 14 and a flow chart of FIG. 15, hereinbelow. The three clocks 21-23 are oscillated at all times. The CPU 25 changes the clock changeover signal 200 to a value corresponding to one of the clocks 21-23 to be changed over and sends the clock changeover signal 200 to the changeover switch 24. Thus, the changeover switch 24 is changed over to the one of the clocks 21-23 by the clock changeover signal 200 so as to generate an output of the one of the clocks 21-23, whereby changeover to the one of the clocks 21-23 has been completed.
In the known clock changeover apparatus described above, as the number of clock signals to be changed over is increased, clocks corresponding in number to the clock signals are required to be provided and thus, a circuit scale becomes large. Meanwhile, if the clock is formed by a crystal oscillator, such a phenomenon may happen that the clock is not oscillated or is oscillated at different frequencies. Even if only one of the clocks is not oscillated or is oscillated at the different frequencies, the clock changeover apparatus does not function properly. Meanwhile, in the known clock changeover apparatus, it is impossible to confirm whether or not each of the clocks is oscillated at a proper frequency. Furthermore, in case one of the clocks is not oscillated at a proper frequency due to scatter in characteristics of constituent elements of the circuit, it is impossible to make an adjustment for reinstating the clock to the proper frequency.